To remotely charge up a transponder in a RF identification system, the transmit/receive (T/R) unit must transmit a high magnetic field strength. A magnetic field instead of an electric field is used because the energy density is much higher than an in electrical field. The principle at work can be compared to a simple transformer with the T/R unit coil being the primary part and the transponder coil being the secondary part. The magnetic field couples to the transponder from the T/R unit with a large air gap in between. In view of the above description, a magnetic field may be generated with a series combination of a simple coil and generator. However, with this configuration, a high field strength is only generated if many windings are used, because the magnetic field is proportional to the number of windings.
Therefore, in order to generate high currents, resonance is used and a series capacitor can be added to the generator/coil configuration of the T/R unit. In an ideal series resonance circuit, with a high quality factor, the voltage drop at the antenna(coil) and thus the current through the antenna is multiplied by the quality factor, Q. A Q of 100, for example, generates a voltage at the antenna that is 100 times the value applied to the resonance circuit and the current is multiplied by the same value. In this way, high currents yielding high magnetic field strengths are generated.
This magnetic field is oftentimes generated by either a series or parallel resonant circuit in the T/R unit. When an AC voltage with the resonant frequency is applied to the tuned antenna circuit, the resonant circuit behaves as a very low ohmic resistance, i.e. the D.C. resistance of the antenna coil, allowing the coil of the resonant circuit to efficiently transmit the energy applied. At resonance, an ideal series resonant circuit will appear to the output stage to be a short circuit (impedance =0 ohms) which could cause damage to the output stage. Therefore, the driver circuit must have the capability to drive this low impedance. A transformer can be used to adapt the power-stage of the T/R unit to the low impedance of the resonance circuit, to protect the driver circuit and determine the amount of power that is transferred to the resonator circuit via the ratio of windings. If a transformer is not used, the minimum allowed D.C. resistance of the antenna coil must be specified to ensure that the low impedance of the load does not destroy the driver. However, there are also several disadvantages to using a transformer, including high cost and high-volume requirements both of which are undesirable in ever increasingly smaller-size production modules.
A possible configuration of a circuit which eliminates the transformer is shown in FIG. 1. There are many different ways to realize the generation of an AC voltage in the T/R unit and one of the more common methods is through use of a push-pull stage. A push-pull stage can be realized with traditional field effect transistors. These transistors are characterized by a low `on` resistance and thus exhibit low power loss and an ability to handle large currents. In addition, transistors are very cost effective components. The circuit shown in FIG. 1 consists of a push-pull stage, consisting of a series connected transistor pair depicted as switches S1 and S2, and a series resonant circuit, consisting of an inductor L3 and a capacitor C4.
A significant disadvantage of this circuit is that the transistors S1 and S2, have to switch the complete RF current that is generated when an AC voltage with the resonant frequency is applied to the tuned antenna circuit. In high power applications, i.e. 400 volts peak to peak voltage, the large amounts of RF current generated make the transistors very, very hot and increase the chance for transistor breakdown (exceed the maximum specified current value). This may decrease the reliability of the T/R unit and may reduce the effectiveness of the reader transmission. Moreover, a large heat-sink is oftentimes required to reduce the heating, and heat sinks require great amounts of volume. The heating of the transistors may also reduce the maximum ambient temperature of the entire reader as the maximum temperature of other reader components may be limited.